Join as observer / prevent player assignment

[elexis]

Often users would like to join a multiplayer game but only spectate and chat, not play. However the host is often not willing or able to follow the desire expressed in the spammy chat. It would make things easier both to the host and observer if the client could communicate to the server that it doesn't want to be assigned.

There could be a new button in the gamesetup, labeled "Observe" or "Spectate" near the existing "Ready" / "Start Game" and "Cancel" button. Pressing the button should immediately unassign that player and remove the player from the dropdown list. The ready button should be kept invisible for observers, unless they are the host (so that the game can be started).

Refs #3806.

[Imarok]

Move tickets to Game Setup as UI & Simulation got some sub components.